WASHINGTON — The Army Corps of Engineers has "made good progress" in incorporating ecological restoration into its proposal for new, longer locks on the Mississippi River, but it still hasn't made a convincing economic case for doing so, a National Academy of Sciences panel said in a report Wednesday.

It added that the Corps still hadn't paid much attention to inexpensive, nonstructural navigation improvements that could ease current barge traffic.

A committee of the academy’s National Research Council reported that grain exports are unlikely to grow enough to justify a $1.46 billion replacement of aging Mississippi and Illinois river locks, or gateways.

“There are no overwhelming regional or global trends that clearly portend a marked departure from a 20-year trend of steady U.S. grain export levels,” the panel said.

Slideshow: Ride a barge through Mississippi locks
The corps has maintained that the longer locks are necessary for barges to get grain and other commodities to Gulf of Mexico ports quicker. The nation’s leading corn and soybean producers — Minnesota, Wisconsin, Iowa, Illinois and Missouri — rely on the Mississippi River as the cheapest route for shipping to export markets.

It was the second time the research council said the corps had failed to justify the construction. The agency has tried for more than a decade to replace the locks, but the project stalled after a whistle-blower accused the corps of inventing its justification.

The corps started over, and the result is a new plan that would spend as much on restoring the environment as on new lock construction. Older locks are too small for today’s tows, causing costly delays as barges are disconnected, sent through locks, and then lashed back together on the other side.

The National Research Council said in December, and again Wednesday, that the corps still fails to justify the project. The justification “contains flaws serious enough to limit its credibility and value within the policymaking process,” the panel found.

A third report is scheduled for next year.

“There is time to correct these problems,” said John Boland, a Johns Hopkins University professor who headed the committee of scientists and engineers reviewing the corps’ data.

Grain is only half of what travels to the gulf; the rest is construction material, petroleum, coal, fertilizer and other products. The corps’ data on non-grain shipments was more flawed than its grain forecasts, the panel said.